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Dipende da cosa ci fai con AutoCad...una piantina, un palazzo, una città?
Comunque questi sono i requisiti minimi di riferimento:

AutoCAD 2012 32-bit
Microsoft® Windows® 7 Enterprise, Ultimate, Professional, or Home Premium (compare Windows 7 versions); Microsoft® Windows Vista® Enterprise, Business, Ultimate, or Home Premium (SP2 or later) (compare Windows Vista versions); or Microsoft® Windows® XP Professional or Home edition (SP3 or later)
For Windows Vista or Windows 7: Intel® Pentium® 4 or AMD Athlon® dual-core processor, 3.0 GHz or higher with SSE2 technology; for Windows XP: Intel Pentium 4 or AMD Athlon dual-core processor, 1.6 GHz or higher with SSE2 technology
2 GB RAM
2 GB free disk space for installation
1,024 x 768 display resolution with true color
Microsoft® Internet Explorer® 7.0 or later

AutoCAD 2012 64-bit
Microsoft Windows 7 Enterprise, Ultimate, Professional, or Home Premium (compare Windows 7 versions); Microsoft Windows Vista Enterprise, Business, Ultimate, or Home Premium (SP2 or later) (compare Windows Vista versions); or Microsoft Windows XP Professional (SP2 or later)
AMD Athlon 64 with SSE2 technology, AMD Opteron® processor with SSE2 technology, Intel® Xeon® processor with Intel EM64T support and SSE2 technology, or Intel Pentium 4 with Intel EM64T support and SSE2 technology
2 GB RAM
2 GB free space for installation
1,024 x 768 display resolution with true color
Microsoft® Internet Explorer® 7.0 or later

For those both 32 & 64-bit using 3D modeling workflows, you require a little more specifics on the graphics card and a wee bit more CPU speed.
Intel Pentium 4 processor or AMD Athlon, 3 GHz or greater; or Intel or AMD dual-core processor, 2 GHz or greater
2 GB RAM or more
2 GB hard disk space available in addition to free space required for installation
1,280 x 1,024 true color video display adapter 128 MB or greater, Pixel Shader 3.0 or greater, Microsoft® Direct3D®-capable workstation-class graphics car